Immune-mediated autoimmune and inflammatory diseases are a major public health issue. Defining the regulatory mechanisms that normally function to prevent pulmonary inflammation is therefore a key to understanding the etiology of these diseases, and for developing therapeutic strategies to boost these activities in patients. Regulatory T cells (TR) expressing the transcription factor Foxp3 play a critical role in preventing autoimmunity and limiting immune-mediated inflammation. We have shown that during type-1 inflammatory responses, Foxp3+ TR upregulate the Th1-specifying transcription factor Tbx21 (T-bet), and that T-bet expression is critical for proper TR homeostasis and function during Th1-mediated inflammation. Therefore, the goals of this proposal are to determine in detail how loss of T-bet specifically within Foxp3+ TR impacts the initiation, progression and termination of Th1 responses in vivo (Specific Aim 1);analyze at the molecular level how Foxp3 and T-bet combine to control the expression of genes involved in Th1/TR differentiation, homeostasis and function (Specific Aim 2);and to identify the cytokines and cellular signals that control the phenotypic and functional differentiation of different TR subsets (Specific Aim 3). PUBLIC HEALTH RELEVANCE: Understanding how regulatory T cells modulate Th1- and Th17mediated immune responses has clear and direct implications in the clinical application of these cells for the treatment of immune-mediated inflammatory and autoimmune diseases caused by dysregulated Th1 cell responses, such as granulomatous inflammation associated with persistent Mycobacterium tuberculosis infection, hypersensitivity pneumonitis, psoriasis, rheumatoid arthritis, type-1 diabetes and multiple sclerosis.